Heat treating of metal



Patented Sept. 29, 1925.

UNITED STATES PATENT OFFICE.

HEAT TREATING 0F METAL.

No Drawing.

To all whom it may concern:

Be it known that I, .ToI-IN E. BURNS, a citizen of the United States, residing at Indianapolis, in the county of Marion and State of I11diana,have invented a new and useful Heat Treating of Metal, of which the following is a specification.

My invention relates to the heat-treating of metals, and to a composition of matter which is suitable for such treatment and which is liquid throughout a wide range of relatively low heat-treating temperatures so that it can be used as a bath to receive the metal at any temperature in such range.

In carrying out my invention, I use a mixture of alkali salts of the oxy acids of nitrogen, which mixture melts at a lower temperature than does any of its component salts, and remains liquid throughout a much longer temperature range than is the case with any of such component salts and to a higher temperature than the boiling points of some if not all of such salts. This makes it possible to get a liquid bath for the heat-treating of metals throughout a ran e of temperature which is not only wide hut extends downward quite low; to get a bath which is neutral or inert so far as its action on the metal is concerned; to get a bath which can easily be removed from the surface of the metal, to leave the .metal clean; and to get a bath which is safe in operation and is free from danger of flashing, which danger is quite a factor in the use of oil for heat-treatingfi While it has sometimes been attempted to heat the metal parts dry, in heat-treating processes, this has not proved satisfactory because of the,formation of scale and because of the difiiculty, if not the impossibility, of getting a uniform temperature, and in consequence, the preferred method of heat-treating metals involves the mersin of the metal parts in a bath. of something liquid, and heatin the liquid to the desired temperature. Mo ten salts have been used where the heat treatment at high temperatures was used, and in some cases at temperatures as low as around 600 F. For instance, either sodium nitrate or potassium nitrate is liquid between temperatures of about 600 F. and 850 F., and those salts have been used separately for heat treatment within that range. Other salts have other temperature ranges, in the main higherthan that just given. The individ- Applieation filed October 5, v1923. Serial No. 666,725.

lower, some other material must be found.

Ordinarily, the salts which are neutral in their action toward metals are not fusible below about 600 F.; so that for lower temperatures it has been customary to use oil. The use of oil, however, is exceedingly dangerous, especially as the temperatures rise, because of the danger that the oil will flash or catch fire. Generally the upper limit 'with oil has been about 600 F. to 650 F.; but the safe upper limit is enerally thought to be less than that. urther, after the treatment with oil it is fairlydifficult toremove the oil film from the surface of the metal part which has been treated, and this has usually involved sand blasting or some similar removal method, at considerable expense.

By making a mixture of. properly chosen alkali salts of the oxy acids of nitrogen, I can get a fairly long temperature range,' and one which includes a great number of desirable heatrtreating tem eratures. The mixture I use may be varie somewhat, according to the heat-treating temperatures desired. l

By using a mixture of 75% sodium nitrate and 25% potassium nitrate, I can maintain a molten state from-about 400 F. to about 1000 F. (These temperatures are only approximate.) This -mixture of sodium and potassium nitrates has a. lower melting point mixture as a bath, it is thus possible to heat- VVhen it is detreat metal to any temperatures within this range of from 400 F. to 1000 F. There is no danger of flashing; this mixture of salts does not attack most metals it is desired to heat-treat, even if it attacks any of them, and certainly does not attack iron, steel, or aluminum; and after the treatment isthrough, any salt which adheres to the surface ofthe metal is removed when the metal part is quenched in water.

Another convenient mixture, and a desirable one especially when a still lower temperature is desiredfis of equal parts of sodium nitrate, potassium nitrate, and sodium nitrite, each com onent thus being 33 of the whole. T is melts at about Another suitable mixture is substantially 25% sodium nitrate, 50% potassium nitrate, and 25% sodium nitrite. This melts at about 325 F, and boils at about 750 F.

The mixtures used may not be-absolute eutectic mixtures; but I believe they measurably approach eutecticity. In any case, they give 'a very desirable range of temperatures, and one that is exceeding y useful for many heat-treating operations.

In using any of these mixtures contemplated by my invention, the mixture is melted,, the metal parts to be treated are placed in the molten mixture, and the mixture is brought to and. maintained at the desired temperature for a suitable time, according to the nature of the heat-treatment desired. Then, ordinarily, the metal pieces are taken out of the bath, and quenched in 20 water; which not only cools the metal but removes effectively any of the salts adhering to the surface thereof.

I claim as my invention 1. A composition of matter for use in heat-treating metals, comprising a mixture containing sodiumnitrate, potassium n1- trate, and sodium nitrite.

2. A composition of matter for/use in heat-treating metals, comprising a'mixture of alkali salts of the bxy acids of nitro en, which composition remains liquid w en quiescent throughout a temperature range extending from about 400 F. o about In witnesswhereof, I have hereunto set my hand at Indianapolis, Indiana, this 29th day of September, A. D. one thousand nine hundred and twenty-three.

JOHN E. BURNS. 

